Vehicle engine displacement may be reduced to conserve fuel and reduce engine emissions. A driver may request similar torque amounts from a small engine as from a larger engine under similar operating conditions. The smaller engine may operate at a higher intake manifold pressure to produce a same amount of torque produced by a larger engine operating at a lower intake manifold pressure. As a result, the larger engine may operate with intake manifold pressure that is lower than atmospheric pressure more frequently than the smaller engine during similar operating conditions. The lower intake manifold pressures, or higher intake vacuum, produced by the larger engine may allow the larger engine to provide more vacuum to vehicle systems that operate with vacuum (e.g., brakes, HVAC vents, etc.). The smaller engine may produce sufficient vacuum at higher barometric pressures (e.g., at lower altitudes), but it may struggle to provide a desired amount of vacuum at higher altitudes where less oxygen concentration may be available.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: operating an engine at idle speed with a homogeneous air-fuel mixture; and richening the homogeneous air-fuel mixture and reducing an engine air flow amount to maintain the engine at idle speed in response to a request for vacuum.
By richening an air-fuel ratio provided to an engine in response to a request for vacuum, it may be possible to provide the technical result of generating additional vacuum within an engine during operating conditions that may not be conducive to generating vacuum. For example, if an engine is idling at a higher altitude and a request for vacuum is provided in response to a low level of stored vacuum, the engine air-fuel ratio may be richened so that the engine provides the same torque as before the engine air-fuel ratio was richened with a lower engine air flow amount. By lowering the amount of air flowing through the engine, the engine may increase a vacuum level of an intake manifold so that the stored vacuum amount may be increased.
The present description may provide several advantages. Specifically, the approach may allow an engine to produce additional vacuum at higher altitudes. Further, the approach may provide additional vacuum without increasing system cost. Additionally, the approach may increase vacuum production without increasing engine emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.